1.) Field of the Invention
The present invention relates to a process for the preparation of a support sheet for a photographic printing paper. More particularly, the present invention relates to a process for the preparation of a support sheet which can be coated with a photographic emulsion layer and display an excellent bonding property without fogging, and thus is useful for producing photographic printing paper having a uniform photographic image-forming property.
2.) Description of the Related Arts
Conventionally, Support sheets produced by coating a surface of a substrate paper sheet having a high size fastness and a high mechanical strength with a white pigment, for example, barium sulfate, i.e., baryta paper sheets, are employed as support sheets for photographic printing paper. Recently, however, the baryta paper sheet has been replaced by a polyolefin-coated support sheet produced by coating both surfaces of a substrate paper sheet with a polyolefin resin. This wide-spread use of the polyolefin-coated support sheet is due to the advantages thereof when compared to the conventional baryta paper sheet. Namely, since the polyolefin is hydrophobic, when a photographic paper sheet is subjected to usual developing and fixing procedures, the polyolefin coated support sheet in the photographic paper sheet is highly resistant to a permeation therein of the developing and fixing solutions, and therefore, the water-washing time and drying time for the treated photographic paper sheet can be significantly shortened, and further, since a permeation of the treating solutions into the polyolefin-coated support sheet cannot occur, the dimensional changes (shrinkage and elongation) of the substrate sheet are significantly reduced, and therefore, the photographic paper sheet can exhibit an excellent dimensional stability.
Nevertheless, the water-proof photographic paper sheet is disadvantageous in that, since the surface of the polyolefin coating layer formed on the substrate sheet is hydrophobic, it is difficult to firmly bond a photographic emulsion layer to the hydrophobic polyolefin coating layer surface through a hydrophilic binder layer comprised of gelatin.
To eliminate this difficulty, a method has recently been adopted in which the hydrophobic polyolefin coating layer surface is activated by applying a corona discharge treatment, flame treatment or chemical treatment thereon, and the activated surface is then coated with the photographic emulsion layer.
Nevertheless, if the surface of the polyolefin coating layer is activated by the above-mentioned treatment, undesirable fogging of the photographic emulsion layer sometimes occurs or the coating thickness of the emulsion layer becomes uneven and the thickness increases at the coating-initiating line. Moreover, the activity of the surface of the polyolefin coating layer obtained by this activation method is reduced with the lapse of time, and therefore, where the photographic emulsion is coated after a long time has passed since the activation treatment, the wettability and adhesiveness of the emulsion layer become poor and the emulsion layer is easily peeled from the surface of the polyolefin coating layer.
To overcome these disadvantages, a method has been adopted in which, after the surface of the polyolefin coating layer is once subjected to the activation treatment, an undercoat layer is formed on the activated surface and the photographic emulsion is coated on the undercoat layer. As the main constituent of the undercoat layer used in this method, there are known (1) gelatin or a composition comprising gelatin as the main component and (2) a composition comprising a hydrophilic resin other than gelatin as the main component.
As the composition (2), latexes of various water-insoluble resins and aqueous solutions of various water-soluble resins have been investigated and it has been found that, when the composition (2) is used, although the low-temperature coagulating step after the coating step need not be carried out, the resultant undercoat layer generally exhibits a poor adhesion to a photographic emulsion layer, and a poisonous gas is generated when some types of resins are used, having an adverse influence on the human body or causing environmental pollution.
When a photographic emulsion layer is formed on the undercoat layer, some types of undercoat layers have an adverse influence on the photographic characteristics, for example, cause fogging of and/or reduce the coating uniformity of the photographic emulsion layer. In this case, it is difficult to select an appropriate composition to be used for the undercoat layer.
In contrast, where the composition (1) comprising gelatin as the main component is used, the formed undercoat layer shows an excellent wettability and adhesiveness and these effects are long-lasting, and the undercoat layer does not have an adverse influence on a photographic emulsion. Accordingly, this type of undercoat layer is widely used.
Where an undercoat layer is formed by using the composition comprising gelatin as the main component, a gelatin-containing coating liquid comprising water as the main solvent is ordinarily used while circulating the coating liquid between a coating apparatus and a storing equipment.
During this circulation, the coating liquid is stirred by an agitator or a pick-up roll in an open system, and excess coating liquid in the coating liquid layer applied in an amount exceeding the necessary amount is scraped off by using a Meyer bar or a blade, or is blown off by an air knife, and is then recovered. Accordingly, air bubbles are inevitably formed in, and violent bubbling also occurs in the coating liquid. Air bubbles in the gelatin coating liquid cause coating unevenness, and when a photographic emulsion is coated on the resultant undercoat layer, an adverse influence is exerted on the coating-initiating line, or coating unevenness occurs in the emulsion layer, and an adverse influence is exerted on the adhesion between the undercoat layer and the emulsion layer.
Accordingly, an anti-foaming agent is usually added to the coating liquid, but the anti-foaming agent forms small pores on the surface of the coating layer when the coating liquid is applied, and such small pores cause a repelling phenomenon, prolongation of the coating-initiating line, or have an adverse influence on the photographic characteristics of the photographic emulsion layer. Accordingly, the use of the anti-foaming agent is restricted. Furthermore, although a defoaming apparatus for removing air bubbles is used, satisfactory effects can not be obtained.
Moreover, where the coating liquid is circulated in the above-mentioned manner, the coating liquid is circulated through a system open to the air, and materials or bacteria floating in the air drop into and are mixed with the coating liquid, and cause defects in the undercoat layer and putrefaction of the coating liquid with a lapse of time. Accordingly, when forming the undercoat layer, a method of preventing contamination by foreign substances in the coating liquid should be taken into consideration. Moreover, an antiseptic effect must be obtained by adding an antiseptic agent or the like.
When the method customarily adopted, for example, the Meyer bar coating method, the blade coating method or the comma coating method is used for coating the undercoat layer-forming coating liquid, sometimes fine coating streaks are formed by the shearing force imposed on the coating liquid. Moreover, where the gravure coating method is adopted, when the gravure roll is separated from the coated surface, coating unevenness in a peculiar pattern is caused. Furthermore, when the air knife coating method is adopted, coating in a peculiar pattern unevenness is often caused by blowing off an excess amount of coating liquid. Therefore, when a photographic emulsion is coated on the undercoat layer formed by any of the above-mentioned conventional methods, many problems or disadvantages are incurred.